1. Field of the Invention
This invention relates to steering systems for articulated vehicles, and particularly to a steering system for effectively combining vehicle articulation with steering via wagon-wheel movement of one axle of the vehicle.
2. Description of the Prior Art
It has become common to articulate certain off-road vehicles such as tractors, graders, scrapers, etc. An articulated vehicle has a smaller turning radius than might otherwise be possible, and is more versatile in rough terrain, especially if the vehicle includes a four wheel drive feature. In such vehicles, articulation is typically achieved by means of a pair of hydraulic cylinders mounted between the two articulating body portions of the vehicle. For turning in one direction, one cylinder is extended while the other is retracted, and for turning in the other direction, the opposite reactions occur. In such systems, steering is controlled by means of a mechanical steering control unit which is tied to the vehicle steering wheel and includes a valve for controlling the flow of pressurized hydraulic fluid to and from the articulation cylinders.
In order to obtain even tighter turning radii for articulated vehicles, articulation has been combined with other steering schemes. For example, U.S. Pat. No. 3,515,235, granted June 2, 1970 to Kamner discloses a combination articulated and Ackerman steering system for vehicles. Similarly, U.S. Pat. No. 3,991,847, granted Nov. 16, 1976 to Unruh shows an articulated vehicle having one axle mounted in a wagon-steer mode in addition to the conventional articulated-steer mode. While attempts have been made to combine various steering schemes on an articulated vehicle, an effective control system for such combined steering arrangements has not been achieved in the prior art. The control and feedback for prior art systems of this type has been dependent upon mechanical or hydraulic linkages, which are susceptible to gradual fading and locational errors due to fluid leakage and mechanical restraints. No system is known which has an effective scheme for efficiently monitoring the various positions of the steering elements with respect to one another and controlling further steering of the vehicle in response thereto.